LTspice Stability – Is UniversalOpamp2 Useful for Capacitive Drive Applications?

ltspiceoperational-amplifierstability

When using a level 2 UniversalOpamp in a unity gain capacitive drive follower application, I find that it is quite stable.

I decided to compare the results to an actual opamp model of the LM7321. I matched the open-loop gain, the GBW product, the output current, the phase margin. Surprisingly, the actual model shows much inferior stability. The ESR of the cap is 1 Ohm.

Is the UniversalOpamp a bad model for such stability analysis ? Or would you infact trust the UniversalOpamp more than the models which often have strange stuff going on.

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Best Answer

If you'll look at the examples/Educational/UniversalOpamp2.asc you'll see that there are 4 levels for the UniversalOpamp2:

  • level1 is a basic VCCS and an R||C output
  • level2 is a single pole opamp
  • level3a is a two pole opamp and programmable phase margin
  • level3b has an additional dominant pole and delay

You used level2, so a simple opamp with a single pole, compared to the model for the 7322 -- which I don't know how it is (haven't looked), but I'd wager it's more than a single pole. Therefore if you want more meaningful results you should use level3a or level3b. And, yes, the UniversalOpamp2 can stand its own against many opamps, when properly tweaked. In fact, personally (and there are others who agree) I'd recommend it over many models of opamps.

Of course, everything is SPICE, therefore an approximation, even the 7322, so take all the results with some grains of salt.

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